What are normally open and normally closed switches?
Normally open (NO: normally open type) means SPST(Single Pole Single Throw)-NO specification.
Normally closed (NC: normally closed type) means SPST-NC specification.
The switchable type is a SPDT(Single Pole Double Throw) type, and can be switched from NC to NO or NO to NC.
There are three types of circuits (contact types) as shown in the figure below. The main type is SPST-NC, but many snap-action switches also use SPDT (switchable type).
For more information, see "Switch Basics" Basics / Pole & Throw.
The SPDT(switchable type) can be used to check the operation of the switch (for normal operation).
If the SPST-NO is for detection (or activation), the SPST-NO must be cut off at that time. By using this as a check circuit, it is possible to confirm that the switch has operated. Note that the load must be adjusted, for example, by incorporating a resistor in the check circuit.
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